RESUMO
Study of pathogenic mechanisms of bacteria has been facilitated by the discovery of easily identifiable markers that are used to differentiate between virulent and avirulent organisms of a given bacterial species. One of these markers is a congo red binding phenotype. In this study, 55 isolates of shigellae were tested for their congo red binding capacity and the results were compared with those of HEp-2 tissue culture assay. Shigella isolates (85.45 percent) which invade into HEp-2 cells can also absorb congo red dye from the agar medium as well as from the solution. Non-invasive or avirulent Shigella (14.55 percent) cannot bind congo red resulting in white colonies formation. The present study shows that congo red binding capacity of virulent Shigella indicating that this assay can be used as a quick marker and inexpensive alternative to other invasive assays to determine the virulence of these bacteria.